In PowerPoint, slides are sized for an onscreen slide show. Slides in PowerPoint are displayed horizontally with slide numbers starting from one, but if the user wants to change the slide’s size and orientation, there are features in PowerPoint to do so.
Orientation is how the document is displayed; it can either be Landscape or Portrait. Portrait displays the slide in a vertical layout, and Landscape displays the slide in a horizontal layout.
Change Slide size and orientation in PowerPoint
Click the Design tab and click the Slide Size button in the Customize group.
In the Shortcut menu, you can choose to select Standard or Widescreen or Custom Slide Size.
If you choose to select Standard, a dialog box will open with two options, Maximize or Ensure Fit; choose either one.
If you choose to select Custom Slide Size, a Slide Size dialog box will pop up displaying settings to customize your slides’ size.
In the slide size section, choose the size you want from the list box by clicking the drop-down arrow to display the list and chose the size.
In the Width section, you can change the width of the slide.
In the Height section, you can change the height of the slide.
In the Number Slides From section, you can select the number of slide sizes you want to change.
In the Orientation, Slides section, you can choose to display your orientation, slides by Portrait or Landscape.
Then click OK.
You can also find the Slide Size button on the Slide Master tab by clicking the View tab and click the Slide Master button in the Master View group.
Once clicked, a Slide Master tab will pop up on the menu bar.
Select the Slide Size button in the Size group and make changes.
I hope this helps; if you have questions, please comment below.